Bipolar type fuel cells are known which have a ribbed bipolar separator prepared from an impermeable thin plate of graphite.
On the other hand, ribbed electrode substrates for monopolar fuel cells have been developed which have a ribbed surface and a flat surface to be in contact with a catalyst layer. Such an electrode substrate is carbonaceous and porous as a whole.
A typical structure of a unit cell in a conventional monopolar fuel cell using such an electrode substrate is illustrated in FIG. 1. The unit cell is composed of two electrode substrates 1, two catalyst layers 2, a matrix layer 3 impregnated with an electrolyte, and two separator sheets 4 to be in contact with ribs 5 of the substrate 1. Such unit cells are stacked to make a fuel cell. Reactant gases, i.e. hydrogen as a fuel gas and oxygen or air, are fed via channels formed by the ribs 5 and the separator sheet 4 and the gases diffuse in the porous electrode substrate 1 from the ribbed surface to the flat surface to reach the catalyst layer 2 and react there.
For preparing such an electrode substrate, the following methods which have been previously proposed may be available. For example, one method for preparing general electrode substrates has been proposed in Japanese Patent Application Laying Open No. 117649/83, wherein mixtures based on short, carbonaceous fibers are pressed into porous shaped articles. Another method is described in Japanese Patent Publication No. 18603/78, in which a machined paper of carbon fibers is impregnated with an organic polymer solution and made into a porous carbon fiber paper. A still another method for preparing an electrode substrate was proposed in U.S. Pat. No. 3,829,327, wherein a web of carbon fiber is subjected to chemical vapor deposition of carbon to make a porous electrode substrate. All electrode substrates prepared by these methods have a substantially homogeneous monolayer structure.
However, such homogeneous monolayer electrode substrates may exhibit some demerits such as follows: with higher bulk densities of substrates, a sufficiently high limiting current density cannot be obtained due to less diffusion of reactant gases in a fuel cell prepared therefrom and premature reduction of the performance of the fuel cell may occur due to an insufficient amount of electrolytes held in the substrate, in other words, the life of the fuel cell is short; on the other hand, with lower bulk densities of electrode substrates, their electric and thermal resistances will be too high and/or the mechanical strength such as bending strength will be too low.
Moreover, in an electrode substrate with ribs, the section modulus thereof is reduced due to a ribbed surface, which is not flat as seen from FIG. 1, and stress is concentrated at the sharp edges 6 of the ribs 5 resulting in insufficient mechanical strength of the whole electrode substrate. A thick substrate is, therefore, inevitably required in order to obtain a sufficiently strong shaped substrate: that is, the resistance of the substrate against diffusion of reactant gases through the substrate from the ribbed surface to the flat surface is increased. In addition, it is difficult to obtain complete flatness of the top surface of the ribs and the incomplete flatness of the ribs' top causes significantly large electric and thermal contact resistances between the ribs' top surface and a separator sheet. As generally known, such a contact resistance may be occasionally several times larger than the conductive resistance in the substrate, and therefore, a conventional monopolar electrode substrate may cause lack of uniform temperature distribution in a fuel cell and electric current generation efficiency thereof will be low due to said large contact resistance.